Single-slope analog-to-digital conversion may be used as an analog-to-digital conversion method for image sensors. According to this method, a ramp signal may be compared with a pixel signal having a predetermined voltage level and a counted time may be measured until the ramp signal has the same voltage level as the pixel signal and may be converted into a digital signal.
A single column analog-to-digital converter (ADC) may be integrated into a single pixel pitch in a column parallel analog-to-digital conversion method. Accordingly, when the area of a layout and power consumption is considered, the single-slope analog-to-digital conversion method may be used. An image sensor may use CDS, count a signal resulting from CDS, e.g., a difference between a reset signal and an image signal, and output a digital signal corresponding to the count result.